The best way to make PLIP and LP live together is to use kernel modules: you can load plip.o and unload it when you need to print or vice versa. If you do really need to use both PLIP and LP, try the following patches.

Of course there is the standard disclaimer: I received these patches and I put them "as I got them". This means that you try them at your own risk. Anyway, your biggest trouble should be only restore the original files and recompile.